Researches on diseases and disorders related to the brain and nerves are now vigorously carried out. In order to diagnose and treat the diseases and disorders related to the brain and nerves, it is important to measure electrical activities of nerve cells in the brain or the like and to arbitrarily stimulate individual nerve cells.
Synaptic transmission between nerve cells is carried out by (i) direct transmission of an electric signal between the nerve cells or (ii) transfer of an electric charge between the nerve cells, which transfer is mediated by an ionic chemical substance such as Ca2+. In either case, the synaptic transmission takes place by a change in an electric charge of the inside and the cell wall, etc. of each individual cell. Thus, for ease of explanation, the term “electrical activity” herein encompasses transmission of an electric signal, transmission of an action potential, and a change in an electric charge of each individual nerve cell.
Conventionally, the below-described techniques have been conducted for (i) measurement of the brain and (ii) stimulation of a nerve cell:
The X-ray CT scanning (X-ray computed tomography) is a technique carried out as follows: X-rays are emitted to a living subject, and three-dimensional distribution of an amount of X-rays absorbed by the living body is measured with a high resolution.
The magnetic resonance imaging (MRI) is a technique that utilizes nuclear magnetic resonance (NMR) of, e.g., a hydrogen atom in a magnetic field. Namely, the MRI emits, in a strong magnetic field with gradient, an electromagnetic wave to a living subject, detects an electromagnetic wave emitted by the living subject in response to the emission, and three-dimensionally visualizes the state of the hydrogen atoms with use of a computer or the like. In particular, the functional MRI (fMRI) can three-dimensionally detect the phenomenon that oxyhemoglobin in the blood is converted into deoxyhemoglobin. Based on the result, the fMRI can specify an activated region in the brain in which region oxygen is being consumed.
The positron emission tomography (PET) and the single photon emission computed tomography (SPECT) are techniques carried out as follows: Substances (radioactive tracers) each labeled with a radioisotope are administered to a living subject, gamma rays emitted by the radioactive tracers are detected with use of gamma-ray detectors arranged around the living subject, and three-dimensional distribution of the radioactive tracers in the living body is measured with use of a computer or the like. Particularly, if FDG (18F Fluoro Deoxy Glucose), which is a glucose labeled with a radioisotope, is used as the radioactive tracer (FDG-PET), it is possible to visualize the activity of the brain by three-dimensionally measuring distribution of glucose metabolism.
The magnetoencephalography (MEG) is a technique carried out as follows: A number of high-sensitive magnetic sensors such as superconduction quantum interference devices (SQUIDs) are arranged around the head of a subject, and fluctuations in magnetic fields caused by electrical activities of nerve cells are measured therewith. Thus, the MEG can directly observe the electrical activities of the nerve cells. Further, the MEG has a big advantage of being capable of carrying out measurement nondestructively and noninvasively.
The electroencephalography (EEG) includes (i) the scalp EEG, which is noninvasive, and (ii) the electrocorticogram, which is invasive. The scalp EEG is a technique for detecting, with use of a number of electrodes set on the scalp of a subject, changes in an electric potential leaked via the dura mater, the cranial bone, and the scalp, so as to measure an electrical activity of the brain of the subject. Thus, the scalp EEG is simple and noninvasive. The electrocorticogram is a technique for inserting electrodes into the surface and the inside of the brain through craniotomy operation so as to directly measure an electrical activity of the brain.
The optical topography (OT) and the near-infrared spectroscopy (NIRS) are techniques both utilizing the characteristics of near-infrared light that the near-infrared light is easy to diffuse and transmit through a living subject. Namely, each of these techniques uses near-infrared light to observe changes in an oxyhemoglobin-to-deoxyhemoglobin ratio in the brain of a subject, which changes are caused by nervous activity. Specifically, each of these techniques is carried out in the following manner: A plurality of optical elements are set around the head of the subject, near-infrared rays of different wavelengths are emitted to the head, and light reflected by the head is detected. Thus, these techniques are completely noninvasive.
The ultrasonography is a technique carried out as follows: A sound wave is emitted to a living subject, the sound wave reflected by the living subject is detected, and three-dimensional distribution of acoustic impedance in the living subject is measured. The ultrasonography is nondestructive and noninvasive, and has a big advantage of being easy to carry out. Further, by using a sound wave having a shorter wavelength, it is possible to improve the spatial resolution.
Meanwhile, as a technique for giving an electrical stimulation to a nerve cell, the following techniques have been proposed: (i) A technique for inserting an electrode needle into a subject so as to directly give an electrical stimulation to the subject. (ii) Magnetic pulse stimulation (transcranial magnetic stimulation; TMS) that utilizes the principle of electromagnetic induction to directly give an electrical stimulation to a subject by an electromotive force.
The above-mentioned techniques for measurement with respect to a living body (e.g., the brain) are comprehensively explained in Non-Patent Literature 1, for example.